In chromatographic practice, it is not all that often the case that the solutions under investigation are solutions of pure substances in pure solvents. The matrix under investigation is frequently extremely complex, and a direct injection of the sample is out of the question. Essentially, in the preparation of samples for chromatographic analysis, use is made of a combination of HPLC columns which either are manipulated offline in the form of disposable cartridges or solid-phase extraction cartridges or which, in the form of normal separation columns, are brought into the eluent flow at the correct time by means of switching valves. In this connection, especially where the said process is automated, there are, however, some problems.
Known arrangements for the enrichment of compounds of dilute solutions—in which, first, the compounds to be separated, i.e. the sample, are enriched at a high flow rate on a so-called trapping column and in which, in a second step, the enriched sample is eluted from the trapping column and separated by the analytical column—have the problem that, during loading of the trapping column, in some cases the flow is flushed to the analytical column. Furthermore, worse chromatographic results are obtained, because there is already separation at the trapping column.
Some of these disadvantages can be avoided with the known backflush process. FIGS. 1a and 1b present an arrangement for performing a backflush process, wherein the arrangement employs a 10-port valve with ports a to j. The component to be separated is retarded at the head of separation column A (see FIG. 1a) and is eluted in the opposite flow direction and separated at separation column B, as presented in FIG. 1b. Using this method, it is possible to minimize the peak widening. Furthermore, it is not necessary in this case to take into consideration the selectivity differences between separation column A and separation column B.
However, also the known arrangement, presented in FIGS. 1a and 1b, for providing a backflush configuration has its disadvantages. For example, the arrangement shown in FIGS. 1a and 1b does not allow the online performance of offline processes, such as loading, washing with 5% methanol, washing with 60% methanol plus 2% ammonia and eluting with 60% methanol plus 2% acetic acid. An online process would, in this case, have to be restricted to loading and eluting. Only with considerable effort and expense would it be possible for such processes to be performed online (it would be necessary either to employ an additional low-pressure switching valve or to replace the isocratic pump with a gradient pump), it being impossible to assess the problems of computer-aided control. To summarize, therefore, it can be said of the known method illustrated in FIGS. 1a and 1b that no additional cleaning steps are possible at the first separation column A, i.e. poor online extracts, and that, furthermore, an additional isocratic pump is required.
Therefore, the object of the present invention is to provide a simpler and better system and method for sample preparation with which, for example, the backflush process can be performed.